1. Technical Field
The invention concerns phased array systems and more particularly a wide-band all-digital architecture for a phased array system that is capable of operating at high data rates.
2. Description of the Related Art
Phased array antenna systems create directional antenna beams by controlling phase and amplitude relationships (RF weighting) among a plurality of antenna elements that are typically arranged in a line or matrix pattern. Analog implementations of phased arrays have been used for many years in a wide variety of applications. These analog systems commonly make use of digital circuitry for baseband signal processing and control, but rely upon analog circuitry in the front-end RF stages to handle functions such as band pass filtering, and RF weighting of antenna elements.
Although analog phased array systems have many advantages over conventional fixed beam antenna systems, they also suffer from a number of disadvantages. For example, analog phased array systems are typically limited in the number of beams and nulls that can be formed. This is an important consideration for spatially differentiating multiple targets, tracking moving signal sources, or eliminating multiple jammers. With conventional analog phased array systems, the addition of more beams or nulls typically requires changes at the hardware and software level. These systems also tend to be RF unique in that each unit must be individually adjusted to compensate for differences in phase and amplitude in the RF circuitry from unit to unit. These problems are compounded by the inherent expense associated with analog RF systems.
All-digital phased array designs have long been considered desirable because they have the potential to overcome many of the problems of analog systems. Such systems can produce a nearly unlimited number of beams, are easily provided with additional beam-forming capability through software upgrades, have the ability to provide multiple nulls in the antenna pattern to thwart multiple jammers and can provide precise angle of arrival information. Such all-digital phased array systems also have the potential to provide a significant cost reduction as compared to analog systems.
Despite the clear advantages offered by the all-digital phased array, these systems have generally been considered impractical for wide band systems that operate at high data rates. This has primarily been due to limitations of existing technology. Systems using conventional covariance matrix techniques, a large number of array elements and high data rate signals, require many gigaflops of data to be processed. This limits the practicality of digital arrays to low data rates. In general, Analog to Digital converters (A/D""s), Application Specific Integrated Circuits (ASICS), and Digital Signal Processors (DSP""s) simply have not been available to meet the demands of an all-digital phased array operating in such an environment.
As with any complex system, there are a variety of architectural and processing options that can be adopted for implementing a digital phased array. However, one difficulty that has been confronted in this area is the selection of an appropriate architecture that can be combined with existing component technology that will permit the realization of a true all-digital phased array. Accordingly, a challenge remains to develop an all-digital phased array capable of operating at high frequency, wide bandwidth and high data rates using available component technology.
Notably, adaptive phased arrays are often used to form simultaneous multiple beams pointed toward desired signals and simultaneous multiple nulls pointed toward undesired signals. A typical system application might include reception and transmission of signals from/to multiple satellites or multiple airborne vehicles. In adaptive arrays where the number of elements is large, for example 100xe2x80x2s or 1000xe2x80x2s, the potential number of adaptive loops is very largexe2x80x94typically equal to Nxe2x88x921 where N is the number of elements in the array. The effective number of loops may be reduced by dynamic range effects, polarization rotation, element pattern, multi-path and array shadowing (etc.) effects, however, there are typically many more adaptive loops than required in practice.
For example, in a typical communications scenario there may be one to four desired signals and a few interfering signals. If all of the elements are utilized in deriving a covariance matrix for weight control, the processing for digital versions of the arrays and the hardware for analog versions of the array is prohibitive or, at least, not affordable for the reasons outlined above. The fundamental issue is to selectively control the type and number of weights utilized so as to optimize the array performance in a given real world environment. The objective of this invention is to fulfill this need via intelligent control of cascaded processing that greatly simplifies both the adaptive weighting and control.
The invention concerns a method and apparatus for cascaded processing of signals in a phased array antenna system in which a plurality of antenna elements are configured as a plurality of sub-arrays. The method is designed to more effectively make use of available received signals to reduce interference from at least one undesired signal.
The process can begin by selectively applying a weighting factor to each of the antenna elements to form a plurality of sub-array beams, each pointed in a selected direction. The weighting factor can be selected exclusively amplitude, exclusively phase, time-delay or complex (phase and amplitude) weights associated with each the antenna element.
For each sub-array, an output from each antenna element in the sub-array can be combined to produce a sub-array output signal. Subsequently, the sub-array output signals can be selectively weighted and combined. In particular, the sub-array output signal received from one of the sub-arrays can be combined with a sub-array output signal from a second one of the sub-arrays in a fully adaptive process.
Subsequently, the system can estimate an angle-of-arrival direction for a signal-of-interest (xe2x80x9cSOIxe2x80x9d) and at least one signal-not-of-interest (xe2x80x9cSNOIxe2x80x9d). The estimating step as described herein can also include estimating an incident power for at least one of the SOI and the at least one SNOI. The estimating can be based on blind source separation (BSS) techniques, a priori knowledge, or direction information of signals learned during system operation.
Based on this estimating step, the system can calculate a new set of weighting factors for controlling one or more of the sub-array beams to improve the signal-to-noise plus interference ratio obtained for the SOI in the array output signal. The calculating step can include calculating a surrogate covariance matrix based solution for at least one of the sub-arrays. This new set of weighting factors is used to selectively control the weighting factors for the one or more sub-array beams. Adjusting the weighting factors for the sub-arrays can result in re-pointing the sub-array beams, and the production of sub-array beam patterns comprising regions of relatively higher and lower gain. In either case, the intent is to improve the signal-to-noise plus interference ratio.
According to a further embodiment, the system can selectively apply one or more alternate weighting factors to each of the antenna elements in one or more of the sub-arrays. The alternate weighting factors are used to independently form alternate sub-array beams using the antenna elements. An output from each of the antenna elements using the alternate weighting factor can be combined to produce one or more alternate sub-array output signals. Selectively weighting and combining one or more of the sub-array output signals with the alternate sub-array output signals in a fully adaptive process can then further improve the signal-to-noise plus interference ratio.
The invention can further include estimating an angle-of-arrival direction for a second SOI and one or more SNOI. A new set of weighting factors can be calculated for controlling the alternate sub-array beams to improve a second signal-to-noise plus interference ratio obtained for the SOI in the alternate array output signal.